Stock Based Compensation - Schedule of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Stock Options
Stock Options, Outstanding Beginning Balance	10,176,643
Stock Options, Granted	1,411,650
Stock Options, Exercised	(607,655)
Stock Options, Forfeited and expired	(194,135)
Stock Options, Outstanding Ending Balance	10,786,503	10,176,643
Stock Options, Vested and exercisable	7,808,020
Weighted Average Exercise Price
Weighted Average Exercise Price, Outstanding Beginning Balance	$ 7.19
Weighted Average Exercise Price, Granted	17.25
Weighted Average Exercise Price, Exercised	5.59
Weighted Average Exercise Price, Forfeited and expired	10.77
Weighted Average Exercise Price, Outstanding Ending Balance	8.53	$ 7.19
Weighted Average Exercise Price, Vested and exercisable	$ 6.51
Weighted Average Remaining Contractual Term
Weighted Average Remaining Contractual Term, Outstanding	6 years 7 months 20 days	6 years 9 months 29 days
Weighted Average Remaining Contractual Term, Vested and exercisable	5 years 9 months 18 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Outstanding	$ 75,562	$ 88,615
Aggregate Intrinsic Value, Vested and exercisable	$ 70,511

Share based compensation arrangement by share based payment award options intrinsic value.

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Share based compensation arrangement by share based payment award options weighted average remaining contractual term.

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For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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Weighted average price of options that were either forfeited or expired.

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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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- Definition

Number of options outstanding, including both vested and non-vested options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Number of fully vested and expected to vest exercisable options that may be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of share options (or share units) exercised during the current period.

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